Soft magnetic materials used for various transformers, reactors, choke coils, noise reduction parts, laser power sources, magnetic pulse power devices for accelerators, motors, power generators, etc. include silicon steel, ferrite, amorphous alloys, Fe-based, nano-crystalline alloys, etc. Silicon steel is inexpensive and has a high magnetic flux density, but it suffers large core loss in high-frequency applications, and it cannot be easily made thin unlike amorphous ribbons. Because of a low saturation magnetic flux density, ferrite is easily saturated magnetically in high-power applications with large operation magnetic flux densities. Co-based amorphous alloys are expensive and have as low saturation magnetic flux density as 1 T or less with practical compositions, providing large parts when used for high-power applications. In addition, because of thermal instability, the Co-based amorphous alloys change with time, resulting in increased core loss.
As an amorphous alloy based on inexpensive Fe, JP 5-140703 A discloses an Fe-based, amorphous alloy for transformer cores, which has a composition represented by (FeaSibBcCd)100-xSnx, wherein a=0.80-0.86, b=0.01-0.12, c=0.06-0.16, d=0.001-0.04, a+b+c+d=1, and x=0.05-1.0, by atomic %, and has excellent soft magnetic properties (large magnetic flux density, low coercivity and a good squareness ratio). However, the theoretical upper limit of a saturation magnetic flux density determined by the interatomic distance, the coordination number and the Fe concentration is as low as 1.65 T, and this Fe-based, amorphous alloy has large magnetostriction, characteristics deteriorated by stress, and a poor S/N ratio in an audible frequency band. To improve the saturation magnetic flux density of the Fe-based, amorphous alloy, it was proposed to substitute part of Fe with Co, Ni, etc., but its effect is small despite high cost.
JP 1-156451 A discloses a soft-magnetic, Fe-based, nano-crystalline alloy having a composition represented by (Fe1-aCoa)100-x-y-z-αCuxSiyBzM′α, wherein M′ is at least one element selected from the group consisting of Nb, W, Ta, Zr, Hf, Ti and Mo, and a, x, y, z and a are numbers (by atomic %) meeting the conditions of 0≦a≦0.3, 0.1≦x≦3, 3≦y≦6, 4≦z≦17, 10≦y+z≦20, and 0.1≦α≦5, 50% or more of its structure being composed of crystal grains having an average diameter of 1000 Å or less. However, because this Fe-based, nano-crystalline alloy is obtained by forming an amorphous alloy and then finely crystallizing it by a heat treatment, it has only a saturation magnetic flux density of about 1.5 T.
As a method for producing an Fe-based, nano-crystalline alloy ribbon, JP 2001-252749 A discloses a method of pouring an Fe-based alloy melt containing 10 atomic % or less of B onto a cooling roll, stripping the resultant amorphous ribbon from the cooling roll in a temperature range of 100-300° C., and then annealing it at the crystallization temperature or higher (for instance, 550° C.). However, JP 2001-252749 A fails to describe the heat treatment atmosphere. Because quenching and stripping are conducted in an inert gas atmosphere, it is reasonable to presume that the heat treatment also is conducted in an inert gas atmosphere.
Japanese Patent 3,639,689 discloses a method for producing an Fe-based, nano-crystalline alloy ribbon comprising ejecting an alloy melt having a composition represented by (FeaSibBcCd)100-xPx, wherein a, b, c and d are numbers (by atomic %) meeting the conditions of 70≦a≦86, 1≦b≦19, 7≦c≦20, 0.02≦d≦4, and a+b+c+d=100, and x is percentage by weight meeting 0.003≦x≦0.1, onto a moving, cooling substrate through a slot nozzle to quench it to form an amorphous ribbon of 30 μm or less in thickness, and annealing it; an average cooling speed from the melting point of the alloy to a temperature range of 150-320° C. being at least 103° C./second, and the ribbon being stripped from the cooling substrate when reaching the temperature of 150° C.-320° C. The annealing was conducted at 360° C. for 1 hour. Japanese Patent 3,639,689 also fails to describe the annealing atmosphere. Because quenching is conducted in an inert gas atmosphere, it is reasonable to presume that the annealing also is conducted in an inert gas atmosphere.
JP 2006-40906 A discloses a method for producing a soft magnetic alloy ribbon comprising quenching an Fe-based alloy melt to form a 180°-bendable ribbon having a mixed phase structure in which an α-Fe crystal phase having an average diameter of 50 nm or less is precipitated in an amorphous phase, and annealing the ribbon at a temperature higher than the crystallization temperature of the α-Fe crystal phase. However, this soft magnetic alloy ribbon unsatisfactorily has a saturation magnetic flux density of about 1.6 T.
JP 2008-231533 A discloses a soft-magnetic, Fe-based alloy ribbon having a composition represented by Fe100-x-yAxXy, wherein A is Cu and/or Au, X is at least one element selected from the group consisting of B, Si, S, C, P, Al, Ge, Ga and Be, x and y are numbers (by atomic %) meeting the conditions of 0≦x≦5, and 10≦y≦24, and having a matrix phase structure in a depth of more than 120 nm from the ribbon surface, in which body-centered-cubic crystal grains having an average diameter of 60 nm or less are dispersed at a volume fraction of 30% or more in an amorphous phase, and an amorphous layer in a depth within 120 nm from the ribbon surface. This soft-magnetic, Fe-based alloy ribbon is produced by quenching an Fe-based alloy melt to form an Fe-based alloy ribbon having fine crystal grains having an average diameter of 30 nm or less precipitated at a volume fraction of less than 30% in the amorphous phase, and annealing this Fe-based alloy ribbon at an average temperature-elevating speed of 100° C./minute or more in a temperature range of 300° C. or higher. This soft magnetic Fe-based alloy ribbon has a high saturation magnetic flux density of 1.7 T or more and low coercivity.
JP 2008-231533 A describes that the heat treatment can be conducted in the air, in vacuum, or in an inert gas such as Ar, nitrogen, helium, etc., particularly preferably in the inert gas. However, there is no Example conducting the heat treatment in the air. When the Fe-based alloy ribbon having fine crystal grains having an average diameter of 30 nm or less precipitated at a volume fraction of less than 30% in the amorphous phase is annealed in the air, too thick an oxide film is formed on the surface. As a result, it has high insulation, but its important saturation magnetic flux density tends to be lower.